Non-static cogging brushless DC motors employ a smooth ferromagnetic stator structure with the coil windings preferably located in the air gap between an inner magnetic rotor and the stator structure. The problem with this construction is that by removing the teeth normally present in a toothed stator structure, the coil-locating and supporting structures have been removed. In general, the coils inserted into the air gap depend upon application of a thermosetting resin ("potting") to hold the coil windings in place. Because the windings must be hand-assembled, the coil position may shift during handling before the plastic resin can be inserted. Misalignment of the coils will result in excessive motor current, variations in the torque produced, and mechanical vibration of the motor.
The problem of reliably supporting the winding coils prior to potting has led to a series of techniques which are labor-intensive and not conducive to automatic assembly methods, including the use of non-magnetic winding pre-forms. U.S. Pat. No. 5, 265,323 to Odell describes a tubular coil form having radially-extending teeth with sidewalls that extend substantially the entire axial length of the coil-forming support member. Thus, the coils are supported by tooth-like structures in the normal manner; but because the tooth material is non-conductive and non-magnetic, no static cogging occurs. The problem with the Odell coil form is that the coil-form tooth structure in the air gap would displace too much volume, thus limiting the use of the maximum amount of coil wire. Ideally, the air gap is completely filled with coil wire. Furthermore, there is no possibility of flaring the end turns after winding so that a rotor can be inserted into the center of the coil structure.
U.S. Pat. No. 4,259,603 issued to Uchiyama et al discloses the use of opposing coil-locating collars at opposite ends of a cylindrical structure to constitute a coil form. The Uchiyama teachings do not demonstrate or suggest the use of retaining the wound coils between inner and outer cylinders. Furthermore, the use of two collars prevents the possibility of applying an outer cylinder to independently support the coils. U.S. Pat. No. 4,818,911 to Taguchi et al shows the use of inner and outer cylinders located about stator coils; however, the coils are not supported by the cylinders, but rather by a separate, self-supporting coil form. The outer cylinder is not applied directly against the coil windings, nor is that structure possible to achieve with the Taguchi coil form.
There is therefore a need in the motor coil assembly arts to create a self-supporting coil winding pre-form which securely holds the wound stator coil in proper alignment and protects it against damage. There is further a need in the art for a structure and method creating an air-gap-positioned coil assembly which permits the use of high-speed, automated coil-winding equipment. There is also a need for a low cost, coil-winding process which will provide a brushless DC motor with high performance.